Technology for noncontact power feeding such as electromagnetic induction has been used in shavers, electrical toothbrushes, and the like. In recent years, there has been a wide interest in the technology that transmits electric power by use of magnetic resonance. In the magnetic resonance power transmission system, a coil (i.e., LC resonance coil) having a resonance function based on capacitor and inductor is provided on each of the transmission side and the reception side. The LC resonance coil provided on the transmission side and the LC resonance coil provided on the reception side have a common resonance frequency. Electric power is transmitted from the transmission side to the reception side via a high-frequency alternating-current magnetic field that oscillates at this resonance frequency. With this arrangement, highly efficient power transmission is achieved even when the distance between a power transmitter and a power receiver is increased or when the sizes of coils are different between a power transmitter and a power receiver.
The magnetic resonance power transmission system typically uses a four-coil configuration. On the transmission side, alternating-current electric power is supplied from an oscillator to a first coil via wire connections, and is then transferred via electromagnetic induction from the first coil to a second coil serving as a resonance coil. Power is transmitted via magnetic resonance from the second coil serving as a transmission-side resonance coil to a third coil serving as a reception-side resonance coil. On the reception side, power is transferred via electromagnetic induction from the third coil serving as a resonance coil to a fourth coil, and, then, alternating-current electric power is supplied from the fourth coil to a power receiving circuit via wire connections. The first coil and the second coil on the transmission side may be combined into one coil, and the third coil and the fourth coil on the reception side may be combined into one coil. Namely, at least one of the transmission side and the reception side may have a one-coil configuration, thereby using three coils in total in the entire system or using two coils in total in the entire system.
A power receiving apparatus that can receive power wirelessly generally has a power-receiving-purpose coil and a magnetic shield. Internal circuitry in the power receiving apparatus tends to have metal materials. Such metal materials may consume some of the energy of an electromagnetic field used for power transmission, resulting in a drop in power transmission efficiency. In order to prevent this, the magnetic shield is disposed between the internal circuitry and the power receiving coil that is situated close to the external surface of the power receiving apparatus. This magnetic shield is composed of a material having large permeability (i.e., the real part of permeability) and small magnetic loss (i.e., the imaginary part of permeability), so that the magnetic field extends along the magnetic shield. The magnetic field thus does not reach the metal materials provided inside the power receiving apparatus, so that power loss is avoided.
The fact that the power receiving coil is disposed at a certain position on the external surface of the power receiving apparatus with the magnetic shield being situated on an immediate inner side thereof means that power cannot be received properly unless the face having the power receiving coil is pointed toward the direction of a power transmitter. When the face having the power receiving coil is pointed opposite the direction of the power transmitter, the magnetic shield and metal materials inside the power receiving apparatus come in the way between the power transmitter and the power receiving coil. In such a case, the magnetic energy hardly reaches the power receiving coil. When the face having the power receiving coil is pointed perpendicularly to the direction of the power transmitter, also, the magnetic field and metal materials inside the power receiving apparatus obstruct the magnetic field, resulting in a large drop in power transmission efficiency.
In order to cope with the above-noted problems, a plurality of power receiving coils may be provided at more than one position on the external surfaces of the power receiving apparatus. A plurality of power receiving coils may be directed to respective, different directions, so that power coming from different directions can be efficiently received. In other words, the position of the power receiving apparatus that ensures efficient power reception is not limited to one position relative to the power transmitter, and any one of the different positions can ensure efficient power reception. Directions and numbers of the power receiving coils may be designed carefully, so that efficient power reception is achieved all the time irrespective of the position of the power receiving apparatus relative to the power transmitter.
The use of a plurality of power receiving coils means that these power receiving coils are connected to a common load (i.e., battery or the like). For example, alternating-current powers from the power receiving coils may be rectified by respective rectifying circuits. Obtained direct-current powers may then be coupled to the common load via a switching circuit. Alternatively, alternating-current powers from the power receiving coils may be coupled to a single rectifying circuit via a switch circuit. Rectified direct-current power may then be supplied to a single load. Either configuration ends up using a switch circuit, and also ends up using a control circuit for controlling switching operations. Further, the configuration that uses a plurality of rectifying circuits ends up providing duplicating circuits. Developing a circuit for controlling switching operations and an increase in the number of circuit components result in a cost increase with respect to the power receiving apparatus. Moreover, an increase in the number of circuit components also leads to a size increase with respect to the power receiving apparatus.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2010-63245